U.S. patent application number 15/111594 was filed with the patent office on 2016-11-17 for method for producing chromium-containing multilayer coating and a coated object.
The applicant listed for this patent is SAVROC LTD. Invention is credited to Juha Miettinen, Jussi Raisa.
Application Number | 20160333493 15/111594 |
Document ID | / |
Family ID | 53542451 |
Filed Date | 2016-11-17 |
United States Patent
Application |
20160333493 |
Kind Code |
A1 |
Miettinen; Juha ; et
al. |
November 17, 2016 |
Method for Producing Chromium-Containing Multilayer Coating and a
Coated Object
Abstract
To produce a chromium-containing multilayer coating on an
object, alternate layers of nickel phosphorus alloy and trivalent
chromium are deposited on the object until a desired thickness of
coating has been reached. The coated object is then subjected to
one or more heat treatments to improve the mechanical and physical
properties of the coating and to produce multiphase layers
comprising layers containing crystalline Ni and crystalline
Ni.sub.3P and layers containing crystalline Cr.
Inventors: |
Miettinen; Juha;
(Hiltulanlahti, FI) ; Raisa; Jussi; (Kuopio,
FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SAVROC LTD |
Kuopio |
|
FI |
|
|
Family ID: |
53542451 |
Appl. No.: |
15/111594 |
Filed: |
January 15, 2014 |
PCT Filed: |
January 15, 2014 |
PCT NO: |
PCT/FI2014/050030 |
371 Date: |
July 14, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 16/44 20130101;
C23C 28/021 20130101; C23C 28/023 20130101; C25D 5/50 20130101;
C25D 3/06 20130101; C23C 18/32 20130101; C23C 16/45525 20130101;
C25D 5/14 20130101; C23C 28/02 20130101; C23C 18/1692 20130101;
C23C 18/1653 20130101; C23C 28/42 20130101; C25D 3/562 20130101;
C23C 14/24 20130101 |
International
Class: |
C25D 5/14 20060101
C25D005/14; C25D 3/56 20060101 C25D003/56; C25D 5/50 20060101
C25D005/50; C23C 16/455 20060101 C23C016/455; C23C 18/16 20060101
C23C018/16; C23C 28/02 20060101 C23C028/02; C23C 14/24 20060101
C23C014/24; C23C 16/44 20060101 C23C016/44; C25D 3/06 20060101
C25D003/06; C23C 18/32 20060101 C23C018/32 |
Claims
1. A method for producing a chromium-containing multilayer coating
on an object, comprising the steps of: depositing a layer of
nickel-phosphorus alloy (NiP) on the object; electroplating a layer
of trivalent chromium on the object; repeating said steps one or
more times in order to produce a multilayer coating containing two
or more alternate layers of nickel-phosphorus alloy and chromium;
and subjecting the coated object to one or more heat treatments to
amend the mechanical and physical properties of the coating and to
produce multiphase layers containing crystalline Ni and crystalline
Ni.sub.3P and multiphase layers containing crystalline Cr.
2. A method according to claim 1, wherein at least one of the
chromium layers is deposited next to a NiP layer and during the
heat treatment at least one multiphase layer is produced that
contains crystalline CrNi.
3. A method according to claim 1, wherein at least one intermediate
layer is deposited on the object between the layers of NiP and Cr,
the intermediate layer consisting of a metal or metal alloy other
than NiP or Cr.
4. A method according to claim 3, wherein the intermediate layer
consists of nickel, copper or molybdenum, or an alloy containing
any of them.
5. A method according to claim 4, wherein a strike layer is
deposited between the chromium layer and the NiP layer.
6. A method according to claim 5, wherein the strike layer consists
of sulphamate nickel, bright nickel, titanium, or any other
suitable material.
7. A method according to claim 1, wherein the NiP layers are
produced by electrodeposition or electroless deposition.
8. A method according to claim 1, wherein the phosphorus content of
the nickel phosphorus alloy is 1-15 w-%, preferably 3-12 w-%, more
preferably 5-9 w-%.
9. A method according to claim 1, wherein the temperature in the
heat treatments is 200-1000.degree. C., preferably 400-750.degree.
C., more preferably 500-700.degree. C.
10. A method according to claim 1, wherein at least two heat
treatments are carried out after the desired number of coating
layers has been deposited on the object.
11. A method according to claim 1, wherein at least one of the heat
treatments is carried out at a temperature between 500 and
700.degree. C.
12. A method according to claim 1, wherein the object to be coated
is of metal and the hardening of the metal of the object is carried
out at the same time as the coated object is heat-treated.
13. A method according to claim 12, wherein the object to be coated
is of steel and the heat treatment is carried out at a temperature
between 750 and 1000.degree. C.
14. A method according to claim 1, further comprising the step of
depositing a top layer on the coated and heat-treated object by
thin film deposition, such as physical vapor deposition, chemical
vapor deposition or atomic layer deposition.
15. A method according to claim 1, further comprising the step of
depositing a top layer on the coated object by thin film
deposition, such as physical vapor deposition, chemical vapor
deposition or atomic layer deposition, before the step of
subjecting the coated object to one or more heat treatments.
16. A coated object comprising a chromium-containing multilayer
coating produced by a method according to claim 1, the coating
comprising multiphase layers containing crystalline Ni and
crystalline Ni.sub.3P and multiphase layers containing crystalline
Cr.
17. A coated object according to claim 16, further comprising at
least one multiphase layer containing crystalline CrNi.
18. A coated object according to claim 16, further comprising at
least one intermediate layer consisting of a metal or metal alloy
other than NiP or Cr.
19. A coated object according to claim 18, wherein the intermediate
layer consists of nickel, copper, or molybdenum, or an alloy
containing any of them.
20. A coated object according to claim 16, further comprising a top
layer produced by thin film deposition, such as physical vapor
deposition or chemical vapor deposition.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method for producing a
chromium-containing multilayer coating on an object. The invention
also relates to a coated object produced by said method.
BACKGROUND OF THE INVENTION
[0002] Chromium coating is widely used as a surface coating for
metal articles because of its high hardness value, attractive
appearance and superior wear and corrosion resistance.
Traditionally, chromium deposition is accomplished by
electrodeposition from a chromium plating bath containing
hexavalent chromium ions as a source of chromium. The process is
highly toxic in nature. Lots of efforts have been made to develop
alternative coatings and coating processes to replace the use of
hexavalent chromium in electroplating. Among those alternative
processes, trivalent chromium electroplating seems to be attractive
due to convenience of fabrication through the use of environmental
friendly and non-toxic chemicals and ability to produce a bright
chromium deposit. However, an industrial scale process giving a
hard and corrosion resistant chromium deposit through an aqueous
trivalent chromium solution is still missing. Among the industry,
there is a hectic need for a well manageable and easy to use
chromium based coating process to replace the current use of
hexavalent chromium in coating.
[0003] Decorative chrome is designed to be aesthetically pleasing
and durable. The thickness of decorative chromium coating is
generally between 0.05 and 0.5 .mu.m. There has been a strong
movement away from hexavalent decorative chromium baths to new
trivalent chromium baths. The trivalent form of chromium is
considered to be less toxic.
[0004] Hard chrome is used to reduce friction, improve durability
through abrasion tolerance and wear resistance, minimize galling or
seizing of parts, expand chemical inertness to include a broader
set of conditions, and as bulking material for worn parts to
restore their original dimensions. Hard chromium coatings tend to
be thicker than decorative chromium coatings. The thickness of hard
chrome can be as high as 200-600 .mu.m. Due to its thickness, the
hardness of hard chrome is usually over 700 HV. Today, hard chrome
is almost exclusively electroplated from hexavalent chromium baths
because of difficulties in reaching desired wear resistance and
hardness by using trivalent chromium baths.
[0005] Many chromium plating processes of prior art are not capable
of producing coatings with a Vickers microhardness value of 2000 HV
or more. Further defects of the known chromium-based coatings are
their inadequate wear and corrosion resistances. Chromium coating
as such is very brittle in character. The number of cracks and
microcracks in a chromium coating increases together with the
thickness of the coating, thus impairing the corrosion resistance
of the coating.
[0006] Deposition of nickel, either by electroless plating or
electroplating, has also been proposed as an alternative to hard
chrome. Drawbacks of nickel plating include deficiencies in
hardness, friction coefficient, wear resistance, corrosion
resistance and adhesion. Nickel plating and hard chrome are not
interchangeable coatings. The two have unique deposit properties
and, therefore, each has its distinct applications.
[0007] In the prior art, several attempts have been made to improve
the corrosion resistance of an object by multi-layer coating.
However, relatively little success has been reported on improvement
of abrasive wear resistance produced by multi-layer coating.
[0008] U.S. Pat. No. 2,859,158 discloses a process for coating
molybdenum with a nickel-chromium diffusion alloy. The process
consists of depositing sequentially a layer of chromium and a layer
of nickel, repeating said sequence of depositions a plurality of
times, and heating the coated molybdenum for 4 hours at a
temperature above 980.degree. C. and substantially below the
melting point of the eutectic of the metals. Chromium layers are
deposited from hexavalent chromium bath. The use of hexavalent
chromium in electroplating is something that should be avoided
today. The heat treatment carried out after coating is quite harsh.
The method is suitable only for coating of molybdenum.
[0009] SE 205488 discloses a coating for an article of ferroalloy,
comprising alternating layers of nickel and chromium, the lowermost
layer being of nickel and the topmost layer being either of nickel
or chromium. Chromium is deposited from hexavalent chromium
containing electroplating bath.
[0010] US 2010/0025255 discloses an electroplating method for
magnesium and magnesium alloy substrate. The method comprises
chemically plating the substrate to form a nickel coating on its
surface and electroplating the substrate to form, in order, a first
nickel coating, a copper coating, a second nickel coating and a
chromium coating on the chemically produced nickel coating.
[0011] Apparently, there is a need for a chromium-based coating
which is able to yield such utmost mechanical properties that
enable replacement of hexavalent chromium baths.
PURPOSE OF THE INVENTION
[0012] The purpose of the invention is to eliminate, or at least
reduce, the problems faced in the prior art. One purpose of the
invention is to avoid the use of hexavalent chromium.
[0013] More precisely, a purpose of the invention is to provide an
environmentally friendly method for producing chromium-containing
multilayer coatings having improved properties, such as a good
sliding wear resistance and an improved corrosion resistance.
SUMMARY
[0014] The coating method according to the present invention is
characterized by what is presented in claim 1.
[0015] The coated object according to the present invention is
characterized by what is presented in claim 16.
[0016] The method for producing a chromium-containing multilayer
coating on an object comprises depositing a layer of
nickel-phosphorus alloy (NiP) on the object, electroplating a layer
of trivalent chromium on the object, repeating said steps one or
more times in order to produce a multilayer coating containing two
or more alternate layers of nickel-phosphorus alloy and chromium,
and subjecting the coated object to one or more heat treatments to
amend the mechanical and physical properties of the coating and to
produce multiphase layers containing crystalline Ni and crystalline
Ni.sub.3P and multiphase layers containing crystalline Cr.
[0017] In this connection, the wording "electroplating a layer of
trivalent chromium" is used to define a process step in which a
chromium layer is deposited from an electrolytic bath in which
chromium is present only in the trivalent form.
[0018] According to one embodiment of the present invention, at
least one of the chromium layers is deposited next to a NiP layer
and during the heat treatment at least one multiphase layer is
produced that contains crystalline CrNi.
[0019] According to one embodiment of the present invention, at
least one intermediate layer is deposited on the object between the
layers of NiP and Cr, the intermediate layer consisting of a metal
or metal alloy other than NiP or Cr. The intermediate layer can
consist of, for instance, nickel, copper or molybdenum, or an alloy
containing any of them. Alternatively, the intermediate layer can
consist of ceramic, such as titanium nitride or chromium nitride,
or diamond like carbon.
[0020] According to one embodiment of the present invention, a
strike layer is deposited between the layers of chromium and NiP. A
strike layer can be used to improve the adhesion between two
layers. The strike layer can consist of, for instance, sulphamate
nickel, bright nickel, titanium, or any other suitable
material.
[0021] The NiP layers can be produced by electrodeposition
(electroplating) or electroless deposition (chemical deposition).
The phosphorus content of the nickel phosphorus alloy can be 1-15
w-%, preferably 3-12 w-%, more preferably 5-9 w-%.
[0022] The temperature in the heat treatments can be
200-1000.degree. C., preferably 400-750.degree. C., more preferably
500-700.degree. C.
[0023] According to one embodiment of the present invention, at
least two heat treatments are carried out after the desired number
of coating layers has been deposited on the object. The coated
object is cooled between successive heat treatments.
[0024] According to one embodiment of the present invention, at
least one of the heat treatments is carried out at a temperature
between 500 and 700.degree. C.
[0025] According to one embodiment of the present invention, the
object to be coated is of metal and the hardening of the metal of
the object is carried out at the same time as the coated object is
heat-treated.
[0026] In one embodiment of the invention, the metal object is of
steel and the heat treatment is carried out at a temperature
between 750 and 1000.degree. C.
[0027] In case the hardening of a metal object is carried out in
connection with a heat treatment of the coated object, it is
possible to subsequently subject the object to annealing or
tempering in a second heat treatment, which is carried out after
quenching.
[0028] It is also possible to subject an already hardened metal
object to a further hardening during the heat treatment of the
coated object even though the metal object had originally been
hardened before the coating.
[0029] According to one embodiment of the present invention, the
method further comprises depositing a top layer on the coated and
heat-treated object by thin film deposition, such as physical vapor
deposition (PVD), chemical vapor deposition (CVD) or atomic layer
deposition (ALD). The top layer can be made of any suitable
material that is able to give the coated surface the desired
properties. Suitable materials comprise, for instance, metals,
metal alloys, ceramics, nitrides (TiN, CrN), and diamond like
carbon (DLC).
[0030] Alternatively, it is possible to produce a thin film
deposited top layer on the coated object before a heat
treatment.
[0031] The coated object according to the present invention
comprises multiphase layers containing crystalline Ni and
crystalline Ni.sub.3P and multiphase layers containing crystalline
Cr.
[0032] In one embodiment of the invention, the coated object
further comprises at least one multiphase layer containing
crystalline CrNi.
[0033] In one embodiment of the invention, the coated object
further comprises at least one intermediate layer consisting of a
metal or metal alloy other than NiP or Cr. The intermediate layer
can consist of, for instance, nickel, copper, and/or molybdenum, or
an alloy containing any of them.
[0034] In one embodiment of the present invention, the coated
object further comprises a top layer produced by thin film
deposition, such as physical vapor deposition (PVD), chemical vapor
deposition (CVD) or atomic layer deposition (ALD). The top layer
can consist of, for instance, metal, metal alloy, ceramic, such as
titanium nitride (TiN) or chromium nitride (CrN), or diamond like
carbon (DLC).
[0035] By means of the method according to the present invention it
is possible to produce chromium-containing multilayer coatings
having excellent mechanical and physical properties. The coating is
hard, dense and ductile, and it has superior wear and corrosion
resistances. The coating method is environmentally friendly and
enables production of thick chromium-containing coatings with
lesser amount of cracks and micro-cracks when compared to
conventional chromium coatings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The accompanying drawings, which are included to provide a
further understanding of the invention and constitute a part of
this specification, illustrate embodiments of the invention and
together with the description help to explain the principles of the
invention.
[0037] FIG. 1a is a micrograph showing Vickers indentation formed
on a single layer coating by a load of 2000 g.
[0038] FIG. 1b is a micrograph showing Vickers indentation formed
on a multilayer coating according to the present invention.
[0039] FIG. 1c is a micrograph showing Vickers indentation formed
on a commercially available hard chrome coating. [0040] FIG. 2a
shows the EDS spectrum of a part of a multilayer coating before
heat treatment, taken from the area of the second Cr layer in a
coating comprising three sequences of NiP--Cr.
[0041] FIG. 2b shows the EDS spectrum of a part of a multilayer
coating after one heat treatment.
[0042] FIG. 2c shows the EDS spectrum of a part of a multilayer
coating after two heat-treatments.
[0043] FIG. 3a shows the EDS spectrum of a multilayer coating
before heat treatment.
[0044] FIG. 3b shows the EDS spectrum of a multilayer coating after
two heat treatments.
[0045] FIG. 3c shows the EDS spectrum of multilayer coating after
two extended heat treatments.
[0046] FIG. 4a is a SEM micrograph corresponding to FIG. 3a.
[0047] FIG. 4b is a SEM micrograph corresponding to FIG. 3b.
[0048] FIG. 4c is a SEM micrograph corresponding to FIG. 3c.
[0049] FIG. 5a shows the surface topography of a hard chrome
coating after an acid test (HCl).
[0050] FIG. 5b shows the surface topography of a coating produced
according to the present method after an acid test (HCl).
DETAILED DESCRIPTION OF THE INVENTION
[0051] The new coating method can be used to produce chromium based
multilayer coatings on objects of different materials, such as
different metals, plastic, ceramic, etc. The new coating can be
used to replace decorative chrome or hard chrome coatings.
[0052] The object to be coated is first subjected to suitable
pretreatment steps, such as chemical and/or electrolytic degreasing
to remove oil and dirt from the surface to be coated, pickling or
etching to activate the surface to be coated, and rinsing as often
needed. Some of these treatments can be repeated between subsequent
coating steps.
[0053] A layer of nickel phosphorus alloy (NiP) can be deposited on
the pretreated object by electroplating, that is, electrical
deposition from an electrolytic bath containing salts of nickel and
phosphorus.
[0054] Alternatively, NiP can be chemically deposited on the
pretreated object (electroless deposition), for instance, from a
solution formulated with sodium hypophosphite as reducing agent.
The result is a nickel film alloyed with phosphorus. The nickel
phosphorus alloy can comprise 1-15 w-%, preferably 3-12 w-%, more
preferably 5-9 w-% phosphorus. The thickness of the NiP layer can
be 0.05-80 .mu.m, preferably 0.5-30 .mu.m.
[0055] A chromium layer can be deposited on the NiP layer by
electroplating from a trivalent chromium containing bath. The
chromium electroplating step can be carried out using any
commercially available Cr(III) bath. One example of an electrolyte
solution that has been used in the trivalent chromium coating step
is the one sold by Atotech Deutchland GmbH under trade name
Trichrome Plus.RTM.. This electrolyte solution comprises 20-23 g/l
trivalent chromium ions and 60-65 g/l boric acid. The working
parameters of the process are: pH 2.7-2.9, temperature
30-43.degree. C. and cathodic current density 8-11 A/dm.sup.2. In
the Trichrome Plus process, graphite anodes are used and additives
are involved to prevent oxidation of trivalent chromium at the
anodes. The thickness of the chromium layer deposited on the NiP
layer can be 0.05-20 .mu.m, preferably 1-10 .mu.m.
[0056] After depositing a layer of NiP and a layer of Cr on the
object, the steps of NiP deposition and Cr deposition can be
repeated as many times as needed to reach the desired thickness of
the coating and/or the desired number of coating layers. If
appropriate, the coated surface can be activated between subsequent
coating steps.
[0057] It is also possible to deposit a strike layer on the
chromium layer prior to depositing the next NiP layer. A strike
layer ensures good adherence of NiP to the chromium layer. To
produce a nickel strike layer, the object is immersed into a nickel
salt-containing bath, through which an electric current is passed,
resulting in the deposition of a nickel layer on the substrate. For
instance, a nickel strike layer can be electroplated on the object
from a nickel sulphamate bath before the electroless deposition of
nickel phosphorus alloy. The thickness of the nickel strike layer
can be, for instance, in the range of 0.1-10 .mu.m.
[0058] It is also possible to deposit an intermediate layer of
another metal or metal alloy on the object between the depositing
the layers of NiP and Cr. Suitable metals for the intermediate
layer comprise, for instance, but not exclusively, copper,
molybdenum, nickel and alloys containing them.
[0059] After the desired number of coating layers has been
deposited on the object, the coated object is subjected to one or
more heat treatments, the purpose of which is to improve the
physical and mechanical properties of the multilayer coating. In
some cases, a further purpose of the heat treatment can be to
harden the metal object under the coating.
[0060] Heat treatments can be carried out at a temperature between
200-1000.degree. C., preferably 400-750.degree. C., more preferably
500-700.degree. C. Preferably, the process comprises two or more
successive heat treatments and the coated object is cooled between
the heat treatments. Heat treatments can be carried out, for
instance, in a conventional gas furnace, in which case the duration
of one heat treatment can be 20-60 minutes. Alternatively, heat
treatments can be carried out by induction, flame heating, or laser
heating. Induction heating is a no-contact process that quickly
produces intense, localized and controllable heat. With induction,
it is possible to heat only selected parts of the coated metal
substrate. Flame heating refers to processes where heat is
transferred to the object by means of a gas flame without the
object melting or material being removed. Laser heating produces
local changes at the surface of the material while leaving the
properties of the bulk of a given component unaffected.
Heat-treating with laser involves solid-state transformation, so
that the surface of the metal is not melted. Both mechanical and
chemical properties of a coated article can often be greatly
enhanced through the metallurgical reactions produced during
heating and cooling cycles.
[0061] When the coated article is an object of metal, it is also
possible to harden the metal of the object during the heat
treatment of the coating. Hardening is a metallurgical process used
to increase the hardness of a metal. As an example, steel can be
hardened by cooling from above the critical temperature range at a
rate that prevents the formation of ferrite and pearlite and
results in the formation of martensite. Hardening may involve
cooling in water, oil or air, according to the composition and size
of the article and the hardenability of the steel. The steel must
contain sufficient carbon to achieve a useful hardening
response.
[0062] After a heat treatment carried out at a steel hardening
temperature (e.g. 750-1000.degree. C.), the coated metal object can
be subjected to annealing or tempering by carrying out a heat
treatment at a lower temperature.
[0063] Hardening of the metal object can be carried out during the
heat treatment of the multilayer coated object even though the
metal object has been subjected to hardening before the coating.
Good results have been achieved by this kind of further hardening
of a coated metal object.
[0064] Finally, a dense top layer can be applied on the coated
object by thin film deposition, such as physical vapor deposition,
chemical vapor deposition or atomic layer deposition. The top layer
can consist of a suitable metal, metal alloy, or ceramic, such as
titanium nitride or chromium nitride, or diamond like carbon (DLC).
The top layer can be deposited on the coated object either before
the heat treatments or after them.
[0065] After at least one heat treatment the multilayer coating
consisting of a plurality of alternating layers of NiP and Cr has
good mechanical and physical properties, such as notably high
hardness values, improved corrosion and abrasive wear resistance
and reduced friction coefficient. Excellent acid resistances have
been measured from multilayer coatings produced according to the
present invention. Individual layers in a multilayer NiP--Cr
coating can be thinner than in a single layer NiP--Cr coating. The
new coating can be used to replace conventional decorative or
functional (protective) chromium coatings in many embodiments.
[0066] The X-ray diffraction spectra (XRD) of the multilayer
coatings indicate the presence of crystalline structures of, for
instance, nickel, chromium, nickel phosphide (NiP.sub.3),
heptachromium tricarbide (Cr.sub.7C.sub.3), eskolaite
(Cr.sub.2O.sub.3) and iron oxide (Fe.sub.3.776O.sub.4) in the
heat-treated multilayer coatings according to the present
invention. Depending on the heat treatment sequence used, the XRD
spectra also indicate the presence of crystalline structures of
nickel oxide (NiO.sub.0.081), chromium carbide (Cr.sub.3C.sub.2) or
isovite (Cr.sub.23C.sub.6).
EXAMPLE 1
[0067] The properties of a double-layer coating of NiP--Cr--NiP--Cr
were compared with the properties of a single-layer coating of
NiP--Cr.
[0068] A first test piece was plated with NiP (thickness 8 .mu.m)
and Cr (4 .mu.m).
[0069] A second test piece was plated with a first layer of NiP (8
.mu.m), a first layer of Cr (4 .mu.m), a strike layer of sulphamate
nickel (2 .mu.m), a second layer of NiP (8 .mu.m), and a second
layer of Cr (4 .mu.m).
[0070] The NiP layers were produced by chemical deposition. The Cr
layers were electrodeposited on the NiP layer from a trivalent
chromium containing bath.
[0071] After coating both test pieces were subjected to similar
heat treatment sequences, consisting of first heating step of 30
minutes at 600.degree. C., cooling to room temperature, and second
heating step of 60 minutes at 600.degree. C.
[0072] The microhardness of the coated test pieces was measured by
Vickers hardness test in micro range using an indenter load of 10 g
(HV 0.01). The tests were carried out according to EN-ISO 6507.
[0073] The sliding wear of the coated test pieces was measured
using modified pin-on-shaft sliding wear tests. The measuring
device resembles a Pin-On-Disk apparatus with the difference that a
rotating shaft is used instead of a rotating disk. The shaft was
rotated at a speed of 300 rpm for 15 minutes or 30 minutes. A ball
made of Al.sub.2O.sub.3 was pressed against the rotating surface
with a load of 200.times.9.81 N or 500.times.9.81 N. The diameter
of the aluminium oxide ball was 6 mm.
[0074] The test results are shown in Table 1. The wear values given
in Table 1 indicate the depth of the pin wear groove created on the
surface under the test conditions.
TABLE-US-00001 TABLE 1 Type of Hardness Slid. wear, .mu.m Slid.
wear, .mu.m coating HV 0.01 500 g/30 min 200 g/15 min Single 2115
29 5.0 NiP--Cr Double 2389 4.0 1.5 NiP--Cr
[0075] After heat treatments both the single-layer coating and the
double-layer coating had an excellent microhardness higher than
2000 HV 0.01. The microhardness of the double-layer coating was
slightly better than the microhardness of the single-layer coating.
The double-layer coating also had significantly better sliding wear
resistance than the single-layer coating.
[0076] The indentation left in different types of coated surfaces
after a Vickers hardness test with a 2000 g load is shown in FIG.
1a-1c. The single-layer coating in FIG. 1a shows a plurality of
cracks around the indentation. The double-layer coating in FIG. 1b
shows essentially no cracking. A commercial hard chrome coating in
FIG. 1c shows widely spread cleavage of coating around the
indentation. FIGS. 1a-1c confirm that a multilayer coating is
tougher than single-layer coatings.
EXAMPLE 2
[0077] The influence of heat treatment on NiP--Cr multilayer
coating was investigated by comparing the properties of a
non-heat-treated multilayer coating with a multilayer coating
subjected to a two-step heat treatment sequence.
[0078] Two test pieces were each plated with following layers: a
first NiP layer (thickness 5 .mu.m), a first Cr layer (2 .mu.m), a
first nickel strike layer (1 .mu.m), a second NiP layer (7 .mu.m),
a second Cr layer (3 .mu.m), a second nickel strike layer (1
.mu.m), a third NiP layer (7 .mu.m), and a third Cr layer (3
.mu.m). The layers of nickel phosphorus alloy were chemically
deposited. The chromium layers were electrodeposited from trivalent
chromium containing electrolyte bath. The nickel strike layers were
electrodeposited from sulphamate nickel bath.
[0079] The first test piece was subjected to a heat treatment
sequence comprising a first heating step of 30 minutes at
600.degree. C., cooling to room temperature and a second heating
step of 60 minutes at 600.degree. C. The second test piece was not
at all heat-treated.
[0080] The morphology of the multilayer coatings was observed by
scanning electron microscopy (SEM). The composition of the coatings
was analyzed by energy-dispersive X-ray spectroscopy (EDS) by
having an electron beam follow a line in a sample image and
generating a plot of the relative proportions of previously
identified elements along the spatial gradient.
[0081] FIG. 2a shows the EDS spectrum of a multilayer coating with
no heat treatment, FIG. 2b shows the EDS spectrum of a similar
coating after one heat treatment step, and FIG. 2c shows the EDS
spectrum of a similar coating after two heat treatment steps. Each
EDS spectrum indicates the elemental composition of the coating in
the area around the second Cr layer, which is located between the
second and the third NiP layer. The steel substrate (not shown) is
located to the left of the measured area and the surface of the
coating (not shown) is to the right of the measured area. EDS
spectra were also measured from the area around the first Cr layer
located between the first and the second NiP layer.
[0082] The EDS analyses verify that two-stage heat treatment of a
multilayer coating increases diffusion between subsequent layers of
Cr and NiP and disperses boundaries between said layers.
EXAMPLE 3
[0083] Three test pieces of stainless steel were coated with
similar multilayer coatings, comprising: a first nickel strike
layer (1 .mu.m), a first NiP layer (7 .mu.m), a first Cr layer (5
.mu.m), a second nickel strike layer, a second NiP layer, a second
Cr layer, a third nickel strike layer, a third NiP layer, and a
third Cr layer.
[0084] The first test piece was not heat-treated at all. The second
test piece was heat-treated in two steps: a first heating step of
30 minutes at 700.degree. C., cooling, and a second heating step of
30 minutes at 400.degree. C. The third test piece was heat treated
as follows: a first heating step of 480 minutes at 700.degree. C.,
cooling, and a second heating step of 480 minutes at 400.degree. C.
In other words, the duration of the heating steps in the third test
was 16 times as long as the duration of the heating steps in the
second test.
[0085] The EDS spectra measured from the coated objects are shown
in FIGS. 3a-3c and the SEM micrographs of the same objects are
shown in FIGS. 4a-4c. The first test piece (FIGS. 3a and 4a) with
no heat treatment shows very clear boundaries between subsequent
layers. The second test piece (FIGS. 3b and 4b), which was
subjected to normal heat treatments (30+30 minutes), shows
increased diffusion between the subsequent NiP and Cr layers,
especially in the outermost layers of the coating. The third test
piece (FIGS. 3c and 4c) with extended heat treatments (480 min+480
min) shows advanced diffusion between different coating layers.
[0086] The X-ray diffraction spectra (XRD) of the heat-treated
samples were measured to get information about the crystalline
structure of the multilayer coating after heat treatment. Most
crystalline materials have unique X-ray diffraction patterns that
can be used to differentiate between materials. The peaks of the
XRD spectrum were identified by comparing the measured spectrum
with the X-ray diffraction patterns of the elements known to be
contained in the coating.
[0087] The XRD spectrum of the test piece subjected to normal
heat-treatments (30 min+30 min) showed peaks that indicate the
presence of the crystalline phases of nickel (Ni), chromium (Cr),
nickel phosphide (Ni.sub.3P), nickel oxide (NiO.sub.0.81),
heptachromium tricarbide (Cr.sub.7C.sub.3), eskolaite
(Cr.sub.2O.sub.3), chromium carbide (Cr.sub.3C.sub.2) and iron
oxide (Fe.sub.3.776O.sub.4).
[0088] The XRD spectrum of the test piece subjected to extended
heat-treatments (480 min+480 min) showed peaks that indicate the
presence of the crystalline phases of nickel (Ni), isovite
(Cr.sub.23C.sub.6), chromium (Cr), nickel phosphide (Ni.sub.3P),
heptachromium tricarbide (Cr.sub.7C.sub.3), eskolaite
(Cr.sub.2O.sub.3) and iron oxide (Fe.sub.3.776O.sub.4).
[0089] The XRD measurements verify the formation of crystalline
structures of nickel, chromium, nickel phosphide, chromium carbides
and chromium oxides during the heat treatment of the coated object.
The measurements also indicate that isovite (Cr.sub.23C.sub.6) is
generated and the amount of eskolaite (Cr.sub.2O.sub.3) and nickel
phosphide (Ni.sub.3P) increases when extending the length of heat
treatments from 30 min+30 min to 480 min+480 min.
EXAMPLE 4
[0090] The acid resistance of the multilayer coating manufactured
according to the present invention was compared with the acid
resistance of conventional hard chrome coatings.
[0091] A steel object was coated with three sequences of NiP--Cr
(10 .mu.m, 7 .mu.m), after which the coated object was subjected to
a first heat treatment of 30 min at 700.degree. C. and a second
heat treatment of 30 min at 700.degree. C.
[0092] Three test pieces with different types of coating were
exposed to 5% sulphuric acid at 100.degree. C. for different time
periods. The edges of the test pieces were protected with varnish.
The test results are shown in Table 2.
TABLE-US-00002 TABLE 2 Duration Corrosion Type of coating min Mass
loss g .mu.m Hard chrome 1 22 0.15 22 Hard chrome 2 2 0.18 7
Heat-treated multi- 23 0 0 layer coating
[0093] Surprisingly, the test piece provided with a multilayer
coating according to the present invention did not show any
corrosion at all after it had been held for 23 minutes in the hot
sulphuric acid.
[0094] Another test piece with a similar multilayer coating and
similar heat treatment was exposed to 35% hydrochloric acid at
20.degree. C. for one hour. The surface topography of the test
piece was compared with the surface topography of a conventional
hard chrome coated test piece exposed to a similar treatment in
hydrochloric acid.
[0095] FIG. 5a shows the surface topography of a hard chrome coated
object and FIG. 5b shows the surface topography of an object coated
according to the present invention.
[0096] It was discovered that the test piece with conventional hard
chrome coating shows large microscopic cracks, whereas the test
piece coated according to the present invention only shows normal
microscopic cracks. This indicates that the new coating is able to
resist hydrochloric acid corrosion significantly better than
conventional hard chrome coating.
EXAMPLE 5
[0097] A surface-hardened metal object was coated with a first
strike nickel layer of 1 .mu.m, a first NiP layer of 3 .mu.m, a
first Cr layer of 4 .mu.m, a second strike nickel layer of 1 .mu.m,
a second NiP layer of 3 .mu.m and a second Cr layer of 4 .mu.m. The
total thickness of the coating was about 16 .mu.m. After this the
object was heat-treated by induction heating.
[0098] First the object was pre-heated by an induction loop with a
power of 26 kW and a speed of 1500 mm/min. Then the temperature of
the object was raised up to 850.degree. C. by induction with a
power of 26 kW and a speed of 1500 mm/min, after which the object
was cooled with water jet.
[0099] The base material was hardened and the hardness of the
coating increased. The Rockwell hardness of the base material after
hardening was 58 HRC and the Vickers microhardness of the coating
was about 1900 HV.
EXAMPLE 6
[0100] A hardenable metal object was coated with a strike nickel
layer of 1 .mu.m, a NiP layer of 3 .mu.m and Cr layer of 4 .mu.m.
The total thickness of the coating was about 8 .mu.m. After this
the object was heat-treated by induction heating in one step.
[0101] The temperature of the object was raised up to 850.degree.
C. by induction with a power of 60 kW and a speed of 1500 mm/min,
after which the object was cooled with water jet.
[0102] The base material was hardened and the hardness of the
coating increased. The Rockwell hardness of the base material after
hardening was 55 HRC and the Vickers microhardness of the coating
was about 1600 HV.
EXAMPLE 7
[0103] An object was coated with a NiP layer of 7 .mu.m and a Cr
layer of 5 .mu.m. The coated object was heated at 700.degree. C.
for 30 minutes. After this a top layer of diamond like carbon (DLC)
was deposited on the coated object by thin film deposition.
[0104] The Vickers microhardness of the coating was over 2000 HV.
The Pin-on-Disc sliding wear of the coated surface was 0 .mu.m
(test duration 210 min, load 500 g and speed 300 rpm). The friction
coefficient of the coated surface was 0.24. The AASS corrosion test
gave a value of over 200 h.
[0105] Alternatively, the top layer could also have been applied
directly on the NiP--Cr coating, in which case the heat treatment
could have been carried out after the thin film deposition
step.
[0106] It is obvious to a person skilled in the art that with the
advancement of technology, the basic idea of the invention may be
implemented in various ways. The invention and its embodiments are
thus not limited to the examples described above; instead they may
vary within the scope of the claims.
* * * * *